Tweak resetElementMap to default to empty defined element map (#12098)

* Tweak resetElementMap tp default to empty defined element map

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/App/ComplexGeoData.h

@@ -265,7 +265,10 @@ public:
      *
      * @return Returns the existing element map.
      */
-    virtual ElementMapPtr resetElementMap(ElementMapPtr elementMap=ElementMapPtr()) {
+    virtual ElementMapPtr resetElementMap(ElementMapPtr elementMap=ElementMapPtr(), bool forceEmpty=false) {
+        if ( ! elementMap && ! forceEmpty ) {
+            elementMap = std::make_shared<Data::ElementMap>();
+        }
         _elementMap.swap(elementMap);
         return elementMap;
     }

tests/src/App/ComplexGeoData.cpp

@@ -444,7 +444,7 @@ TEST_F(ComplexGeoDataTest, saveDocFileWithNoElementMap)
 {
     // Arrange
     Base::StringWriter writer;
-    cgd().resetElementMap();
+    cgd().resetElementMap(nullptr, true);  // Force undefined map
 
     // Act
     cgd().SaveDocFile(writer);